Rarity and Rainbow Dash (After Rarity Investigates, I'd like to see them interact more)
Fluttershy and Applejack (They don't interact all that much in the show, so I'd love to see them work on solving a problem)
Rarity and Pinkie Pie (Again, they don't interact all that much.
Finally, Pinkie Pie and Fluttershy (Two ponies so different with one another, I'd love to see them find common ground with one another)